li {
   list-style: none;
}

li span {
   padding-left: 20px;
   cursor: pointer;
   background: url("../img/add.png") no-repeat center left;
}

li ul {
   display: none;
}

.open {
   background: url("../img/minus.png") no-repeat center left;
}

.open+ul {
   display: block;
}